For the Neutrino Factory and Muon Collider Collaboration, BNL has considered solenoidal magnet systems of several types to capture pions generated by bombarding a mercury jet with multi-GeV protons. The magnet systems generate up to 20 T, uniform to 5% throughout a cylindrical volume 0.15 m in diameter and 0.6 m long. Axially downstream the field ramps gradually downward by a factor of sixteen, while the bore increases fourfold. The steady-state system needed for an accelerator has many superconducting coils and a radiation-resistant insert of mineral-insulated hollow conductor. Less costly, pulsed systems suffice to study pion capture and the effect of a magnetic field on a jet hit by a proton beam. BNL has explored three types of magnets, each with its principal coils precooled by liquid nitrogen. One type employs two sets of coils energized sequentially. Charged in 23 s by a power supply of 5 MVA, the 16ton outer set generates 10 T and stores 28 MJ, from which, in 1/3 s, to charge a half-ton inner coil that adds 12 1/2 T to the 7 1/2 T remaining from the outer set. An alternative design uses 25 MVA to energize, in 1.4 s, a single 3-ton set of coils. The third type bows to budgetary constraints and is more modest in size and performance. A magnet of 2-3 tons generates 10-11 T with only 2 MVA, in a bore big enough (11 cm) to accommodate the jet. It forgoes the field ramp that improves pion retention

Weggel, R. J.

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BNL - 69044CAP-340-NuFact–02CConceptual Design of Pion-Capture Magnetsof up to 15 cm Bore and 20 T Peak FieldRobertJ.WeggelBrookhmenNationalLaborato~, Upton, iVeu’YorkJanuary 2002Pres. at “17’h I.lternational Cofiference en Maqmt Tec.hmloqy (MTl~)”, Ge,?eva,Switzerland, Sept. 24-28, 2001.1Conceptual Design of Pion-Capture Magnets ofup to 15 cm Bore and 20 T Peak FieldRobertJ.WeggellAbstract—For the Neutrino Factory and Muon ColliderCollaboration, BNL has considered solenoidal magnet systems ofseveral types to capture pions generated by bombarding amercury jet with multi-GeV protons. The magnet systemsgenerate up to 20 T, uniform to 5% throughout a cylindricalvolume 0.15 m in dbuneter and 0.6 m long. Axially downstreamthe field ramps gradually downward by a factor of sixteen, whilethe bore increases fourfold. The steady-state system needed foran accelerator has many superconducting coils and a radiation-resistant insert of mineral-insulated hollow conductor. Lesscostly, pulsed systems suffice to study pion capture and the effectof a magnetic field on a jet hit by a proton beam. BNL hasexplored three types of magnets, each with its principal coilsprecooled by liquid nitrogen. One type employs two sets of coilsenergized sequentially. Charged in 23 s by a power supply of 5MVA, the 14-ton outer set generates 10 T and stores 28 MJ,from which, in 1/3 s, to charge a half-ton inner coil that adds12% T to the 7% T remaining from the outer set. An alternativedesign uses 25 MVA to energize, in 1.4 s, a single 3-ton set ofcoils. The third type bows to budgetmy constraints and is moremodest in size and performance. A magnet of 2-3 tons generates10-11 T with only 2 MVA, in a bore big enough (11 cm) toaccommodate the jet. It forgoes the field ramp that improvespion retention.Index terms-cryogenic puke magnet; hybrid magnet;radiation-resistant magnetI. PERFORMANCEPARAMETERSOFPION-CAPTUREMACiNETSOne component of a neutrino factory or muon collider isthe magnet system to capture pions that en-mate when multi-GeV protons in a beam of 1 MW or more bombard a target,such as a mercury jet 0.6 m long tilted 100 mr with respect tothe magnet axis. To capture most of the pions, by bendingtheir trajectories into helices, requires a field of about 20 T ina bore of 0.15 m. To retain the pions, and the muons intowhich they decay, the on-axis field profile a distance zdownstream from the target should approximateB(0)/(1 + kz), with k small, so that particles experiencelittle change in field in any Larmor orbit. The bore shouldincrease inversely with the square root of the field, enclosingconstant flux. The field should fall by an order of magnitudeor so, to convert transverse momentum into longitudinalmomentum, then the field can level out.ManuscriptreceivedSept. 24, 2001. Workperformedunder the auspicesof the U.S.Dept.of Energyundercontractno. DE-AC02-98CHI088.1R. J. Weggelis a researchengineerat BrookhavenNationalLaboratory,Upton,NY 11973-5000,U.S.A.(631-344-2428,e-msil weggel@bnl.gov).II. HYBRfO MAGNETSYSTEMFORACCELERATORThe steady-state magnet system required by a neutrinofactory or muon collider must be a hybrid system, with manysuperconducting coils surrounding a resistive insert, as in Fig.1 [1]. The field is 20 T over the target, ramping downward to1.25 T, a factor of 16, over the next 18 m, the bore growingfourfold to 0.6 m. Its main coil generates a peak field of 14 Ton the axis of a bore of 1.3 m. A precedent for the coil is theInternational Thermonuclear Experimental Reactor’s 140-tonCentral Solenoid Model Coil [2]. In a larger bore, 1.6 m, ithas generated almost as much field, 13 T, storing 600 MJ, thesame as the entire pion-capture magnet. The upstream fivesuperconducting coils employ cable-in-conduit conductor, theconductor of choice for large magnets operating at many kA;smaller, lower-field coils use Rutherford cable.EzMlLc.wlwKm41pEmM41gcciIs•m~—iss%w KrB’iutimwsk-ds———— ————— -——~.s%.—— _———— .—Fig. 1. Crosssection(foreshortenedaxiallyby factorof two)of upstream11m of windingsof steady-statepion-capturemagnet. The systemstores 600MJ: its main coil generates 14T in a 1.3m bore. Insidearea ferromagneticplug (blackT) and mercury target (tilted black rod) and three nested coils ofmineral-insulated hollow conductor. The proton beam is tilted -67 rrrr.Thelong flared tube shows the radial limit of pion retention.To survive the intense radiation from the target, the insertemploys mineral-insulated hollow conductor, as developed byTanaka [3] for the Japan Hadron Facility. Stainless steelreinforces the weak conductor, whose copper is soft forfabricability. Because only about one third of the windingcross section carries current, the coil requires 12 MW togenerate only 6 T. Radiation-resistant hollow conductor withmuch less space devoted to insulation and sheathing mightgenerate the field with half the power. Better yet might be aBitter coil that at 6 MW might generate 7% T. An R&Dprogram to perfect an insulator (presumably ceramic) towithstand high levels of radiation and the hostile environmentof the Bitter magnet could save millions of dollars in powersupplies and superconducting magnets.2III. MAGNET SYSTEM m INDUCTIVEENERGYSTORAGEPart of any R&D program leading to a neutrino factory ormuon collider should be the design and fabrication of amagnet system to study the behavior of a mercury jettraversing a magnetic field and hit with a high-energy protonbeam. Pion capture and retention also are of interest. To thisend BNL has studied three types of magnet systems. Foreconomic reasons none is steady state; all use copperprecooled by liquid nitrogen. The first enlists inductiveenergy storage and a power supply of 5 MVA that BNL mightpurchase. The second draws 18 MJ from the kinetic energy ofthe rotor of the 25-MVA Westinghouse generator that is thebackup power source for BNL’s Alternating GradientSynchrotrons. The third, dictated by budget considerations,employs four standard BNL power supplies in series/parallel(-2 MVA), to generate only 10-11 T, and in a smaller bore(11 cm), and without the field tail that improves pionretention.The frost type of system includes two sets of cryogenicpulse coils [4]. Fig. 2 sketches the cross section of the coils;Fig. 3 plots the time dependence of its current, field,cumulative heating and temperature rise. A 5 MVA powersupply charges the outer set of coils to 16 kA, 10 T in 23 s.Disconnection of the power supply, followed by insertion of a1/4 Q resistor across the terminals of the set, then creates avoltage---initially 4 kV—with which to charge the inner set ofcoils. In about 1/3 s it reaches 10.4 kA, 12.6 T. Meanwhile,inductive coupling and resistive losses discharge the outer setto 12 kA, 7.5 T. A resistor introduced across the terminals ofthe inner set discharges it quickly, to limit its temperaturerise. With a resistor of 385 rnfl (for an initial dischargevoltage of 4 KV, as with the outer set) the inner coil decays to4 kA in 0.8 s; the outer set reaches 4 kA at 2s.A ferromagnetic plug immediately upstream of the nozzlefor the mercury jet improves its entry into the field by halvingthe field gradient. Centered 3 m from the end of the jet maybe a radio frequency cavity, to test its performance in a highradiation environment. If so, water-cooled coils generate the1%-T dc field in which to condition and run the cavity.BLc@WIII —mg~k~z~k:kj~iik.Id~_Jmd————— S& hit OkqmhF&h Afkr-~–_Fy_~Ezzlmmz’- ~ “---==1.F;a@siiiii‘6’”/[1 ]\!%:?m /\,d@xlt3 m/!IFig. 2. Cross sectionof windingsof 20-T piorr-capturemagnet in which a14-tonouter set of pulse coils stores energy to charge a half-ton inner set(black). The ferromagnetic plug (black T) at left halves the magnetic shearon the mercury jet (tilted black rod). Water-cooled coils at right generate 1%Tin the bore of the rf cavity centered 3 m downstream from the jet.2016 . .; .—12840-2.4 -2.0 -1.6 -1i1: outerc2 innerc3 field at4 outers5 inners6 heatin(~-0.8 - 0 0.4 0.8 1,2 1.6Time (in tens ofs for tcO; ins for t>O)Fig. 3. Time dependence of current and temperature rise in inner and outersets of coils of magnet of Fig. 2. Also, combined field and cumulativeheating. Charging the outer set takes 23 s; the inner set, only 1/3 s.IV. MAGNETSYSTEMPOWEREDBY 25 MVA GENERATORA two-stage system can multiply the effective size of itspower supply, but requires many tons of conductor andexpensive switchgear. Thus, it was tantalizing to learn thatone might consider a magnet matched to the 25 MVAWestinghouse generator that once powered the AlternatingGradient Synchrotrons, but now is only a backup. Fig. 4sketches the system. The pulse coils use only 3.2 tons ofcopper, only 22% as much as in the two-stage system. Thesystem is compact, so as not to overtax the -20 MJ energyrating of the Westinghouse generator. Fig. 5 plots the timedependence of the magnet’s current, field, resistance.cumulative heating and maximum temperature rise. Fig. 6plots its on-axis field profile.Fig. 4. Cross section of windings of 20-T pion-captrrre magnet energized by25 MVA Westinghouse generator. The pulse coils weigh 3.2 tons. Theferromagnetic plug (black T) and contouring of the bore of the main coilachieve a field homogeneity of 5% over the length of the mercury jet (tiltedblack rod). Water-cooled coils at right geuerate 1H T in the bore of the radiofrequency cavity centered 3 m downstream from the jet.2ClE12e40/30.4 0.8 1.2 1.6 2.0 ;Time (s)Fig. 5. Time dependence of current, field, resistance, cumulative heatingand temperature rise of magnet of Fig. 4. Charging takes 1.4 s; the totalpulse length is 2.3s.The field is within a few percent of its peak intensity forseveral times as long as with the two-stage system, becausethe peak current density not as high. Retool should take muchless liquid nitrogen, because the cumulative dissipation isonly 6.4 MJ instead of 36 MJ. However, retooling may takelonger, because the peak temperature rise is 50 K, instead of14 K (for the inner set) to 17 K (for the outer set).Distance from downstream end of target region (m)Flg.6. On-axis field of pion-captnre magnet of Fig. 4. The pulse magnetgenerates most of the field everywhere except at the very upstream end,where a ferromagnetic plug contributes 1.2 T and cancels up to half of thefield gradient. Water-cooled coits generate most of the field beyond 2.3 m.V. 10 T, 2 MVA PULSE MAGNET WITHOUTFIELDTAILUnfortunately, as our exploration of alternative magnetsystems progressed, it became more and more clear that allwere beyond our budget. The two-stage system might costwell over a million dollars for the magnet and cryostat, and atleast another million for the new power supply andswitchgear. The Westinghouse system, despite availing itselfof rotating machinery that is free, calls for transformers,rectifiers and bus work to the tune of a least a couple milliondollars. Our budget compels us to settle for less ambitiousperformance, and to employ existing, rather antiquated, BNLpower supplies. The most numerous of these deliver 3.6 kA ateither 125 V or 150 V. BNL electrical engineers consider itfeasible to gang four of these in series/parallel, to give 1.8MW or 2.16 MW, respectively.Fig. 7 sketches the winding cross section of a magnetoptimized for 1.8 MW. For efficiency the magnet has twogrades of conductor, the outer layers with about half thecurrent density of the inner ones. Not shown are the internalcooling passages required for rapid retooling. One generatesthese passages by spacer strips, either axial (if the magnet islayer wound) or radial, like wheel spokes (if the magnet is ofdouble pancakes).1,8MVALN2PukeMagnettoGenerate9,5-10Tthroughout0,11mdiamby0.60mtargetregionaxisFig. 7. Cross-section of windings of magnetProtonbeam(-67mr)withwhich to study thebehavior of a mercury jet traversing a magnetic field and hit by a high-energy proton beam. The coil weighs 2 tons. The outer winding have half thecurrent density of the inner ones. Contouring the inner radius keeps the fieldconstant to 5% throughout the target (tilted black rod).At 2.16 MVA, a magnet of this type can generate 11 T with2.4 tons of copper. A magnet without the conductor gradingrequires about 1/6 more power to generate a given field. Ofcourse, fabrication is simpler. Current density that is invariantwith radius also allows one to consider double-pancakeconstruction, with its virtue of modularity. Pancakes of largerbore near the ends of the magnet can improve experimentalaccess and allow more room for the mercury jet to enter andexit. Alternatively or additionally, pancakes permit gradingthe conductor axially for exquisite field homogeneity, withoutthe complexity of a non-constant bore diameter.Fig. 8 plots the time history of the current, field. resistance.cumulative heating and peak temperature rise. Fig. 9 plots theon-axis field profile.41[EE4~co 1.2 2.4 3.6 4.6 6,0 7.2 6.4Time (s)Fig. 8. Time dependence of current, field, resistance, cumulative heating[~ and tempera~e rise (of the inner win&gs) of the magnet system o~Fig. 7. Charging takes 6.2 s; the total pulse length is 8.4 s. The temperaturerises 25 K by the end of the pulse.Distance from downstream end of target (m)Fig. 9. On-axis field profile of magnet of Fig. 7. The field profile matchesthe desired one very well within the target, bnt declines much too quickly forgood pion retention beyond -0.2 m.Fig. 10 explores the relationship between peak field andpower-supply size for pulse magnets of two bore diametersand field profiles. The smaller bore is the minimum that canaccommodate the tilted target. The larger bore providessuperior experimental access and captures more pions. Thefield profiles labeled “with tail” include a 3 m ramp, asdefined in Part I and plotted in Fig. 9, to improve pionretention. All four lines show that field at frost (<10 T)increases almost as the square root of power, and later (-20T) more like the cube root. The larger bore costs -13% infield; the pion-retention ramp, - 18%.P?/’”“1.6 2 3 4 5 7 10 14Power supply MVAFig. 10. Field vs. power for magnets similar to Fig. 7 (if no field @ or thepulse coil of Fig. 4 (if with field tail). At any given power, decreasing thebore horn 15 cm to 10 cm improves the field by -15%. Abandoning the fieldtail that improves pion retention increases the field by -22%.VI. CONCLUSIONS AND PLANSFOR Fu’ruRE WORKAs one can gather from the frequent references to budgetconstraints, only the final option, based on existing powersupplies, is viable now. The hybrid magnet system must awaitapproval for the design and construction of a NeutiinoFactory or Muon Collider. The two-stage system makesclever and effective use of inductive energy storage, but is tooambitious and costly, probably even if some other departmentat BNL would consent to share the cost of the power supplythat they then would inherit. The design that uses theWestinghouse power supply could be attractive, but only ifthe AGS is willing to share much of the cost of the newtransfon.ners and other components that they would inheritfrom the upgrading of this power supply.The system based on existing BNL power supplies is mostappealing. If cooled to -30 K, it might generate as much as15 T. We have prepared budgetary cost estimates for themagnet and cryostat and are examining magnet stresses, cool-down considerations, and the cost of ganging power supplies.We hope to prepare a detailed design during 2002. If so,fabrication may begin in 2003.VII. REFERENCES[1] R. J. Weggel, C. H. Pearson. B, J. King, “Design study for 20 T, 15 cmbore hybrid magnet with radiation-resistant insert for pion capture,”PACOI, May 2001.[2] R. J. Jayakumur etal.,“The USHT-ITER CS Model Coil programachievements,” IEEE Trans.AppL Supercm.,10:1,2000.[3] K. H. Tanaka et al., “Development of radiation-resistant magnet coilsfor high-intensity beam lines,” IEEE Trans.Mugn. 30, 1994.[4] R. J. Weggel, “Conceptual design of a magnet system to generate 20 Tin a 0.15 m bore, employing an inductor precooled by LN2, IEEETrans cm Magn., Sept. 1999.

Conceptual Design of Pion Capture Magnets of Up to 15 Cm Bore and 20 T Peak Field.

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